Trans-sialidase catalyzes the transference of sialic acid from host to the Trypanosoma cruzi surface. Here, we characterize the sialic acid acceptors of this protozoan parasite as mucin-like molecules, which are anchored to the membrane by glycosylphosphatidylinositol. The mucins isolated from the insect stages differ from the mucins isolated from the mammalian stages in size and reactivity to monoclonal antibodies, suggesting that they are formed by variable polypeptide chains and/or O-linked carbohydrate structures.